Compositions selectively absorbing far to near infrared rays having wavelengths of 600 nm or more have been demanded for their possible application to various fields, but none of the conventional infrared absorbing compositions has been satisfactory.
Near infrared absorbing substances are used in infrared cut filters, such as a safelight filter for near infrared-sensitive photographic materials, an infrared cut filter for human eyes, plastic films for plant growth control, a near infrared cut filter for semi-conductor photoreceptors, e.g., a silicone photodiode (SPD), and the like. Their properties to convert absorbed near infrared radiation to heat are utilized in heat-sensitive color formation or heat-sensitive recording by laser rays, photo disc recording layers, acceleration of ink drying, and the like. Further, they are coated on various light-sensitive materials as an antihalation layer to improve image qualities. In addition, attempts are being made to incorporate them in inks for laser beam-readable cards or inks for ink jet printers. Of these applications, five typical examples are described below in some detail.
(1) Safelight filters for infrared-sensitive light-sensitive materials:
In recent years, many silver halide light-sensitive materials have sensitivity to far to near infrared rays of 700 nm or more have been developed. Whether they are for black-and-white image formation or color image formation, or whether they are of ordinary type, instant type, or heat development type, they exhibit infrared-sensitivity to be used as false color films for exploitation of resources or to be exposed to light of a diode emitting infrared rays. In order to protect these light-sensitive materials from infrared light, safefilters for panchromatic films have been conventionally used.
(2) Plant growth control
Morphogenesis of plants relating to growth and differentiation, such as germination of seeds, growth of stems, development of leaves, formation of flower buds or tubers, etc., is known to be influenced by light and its study is known as photomorphogenesis.
Plastic films which selectively absorb light of 700 nm or more would be useful to control plant growth, for example, in such a manner that crops in a specific stage are covered with a near infrared absorbing film to retard earing-up (see Katsumi Inada, Shokubutsu no Kagaku Chosetsu (Chemical Control of Plants), Vol. 6, No. 1 (1971)).
(3) Heat radiation shielding
Radiation energy of sunlight in the near infrared and infrared regions having wavelengths of 800 nm or more is absorbed in objects and transformed into heat energy. Most part of the radiation energy distribution is concentrated to the near infrared region of from 800 to 2000 nm. Therefore, films which selectively absorb near infrared rays are extremely effective to shield the sun heat and useful to control a room temperature rise while sufficiently letting visible light in. Such films can be applied to horticultural greenhouse glazing, windows of houses, offices, shops or automobiles, aircraft glazing, and the like.
Conventionally employed heat radiation shields include plastic films on which a very thin metal layer is vacuum evaporated, and glass having dispersed therein inorganic compounds, e.g., FeO.
(4) Cut filters for human eyes to protect from harmful infrared rays
Infrared rays contained in the sunlight or light emitted during welding are harmful to human eye tissues. One of main uses of infrared cut filters is protection of human eyes from light containing such harmful infrared rays as in, for example, sunglasses and protective glasses for welders.
(5) Infrared cut filters for semi-conductor photoreceptors
Light-detectors used in automatic exposure meters for cameras, etc. mainly employ a silicone photo-diode (SPD) as a photo-receptor. FIG. 4 shows a specific visibility curve and a graph of spectral sensitivity (i.e., relative outputs) of SPD. For use of SPD in exposure meters, it is necessary to cut the light in the infrared region that is not perceivable by human eyes to make the spectral sensitivity curve of the SPD approximate the specific visibility curve. In particular, the high outputs of SPD in the wavelength region of from 700 to 1100 nm to which human eyes are insensible cause working errors of exposure meters. Accordingly, plastic films which absorb light over the entire infrared region of from 700 to 1100 nm wavelength range would obviously increase the relative light transmittance in the visible region to increase the output of the SPD in that region, leading to marked improvements in the performance of exposure meters. In the conventional photo-detectors of this type, an infrared cut filter made of glass containing an inorganic infrared absorbent is fixed in front of the SPD.
Most of the organic dye type infrared absorbents which have been generally employed are not satisfactory in practical use due to their low light- and heat-resistance.
Further, filters used in the above-described various applications have the respective disadvantages as set forth below.
Conventional safelight filters for panchromatic films applied to use (1) above cannot sufficiently achieve their purpose as a safelight filter for infrared-sensitive photographic materials because they transmit not only green light of high visibility in part, but also a large quantity of infrared light, which causes light fog.
Plastic films having a metal deposit layer or glass having dispersed therein FeO applied to use (2) above strongly absorb light in not only the infrared region but also the visible region, resulting in reduced inner illumination, which causes an absolute decrease in sunshine, which is particularly detrimental in agricultural use.
The glass cut filters containing inorganic infrared absorbents applied to use (5) above are relatively resistant to heat and light, but have a low temperature in the visible region. This disadvantage has been coped with by increasing the sensitivity of the SPD. Increase of SPD sensitivity, however, leads to an increase of leakage current, which causes errors in working as a photo-detector, thus reducing reliability of the photo-detector. Further, the cut filters comprising inorganic substances lack the softness required in the production of photo-detectors, and it is difficult, for the time being, to make improvements on the process of their production. Furthermore, inorganic infrared cut filters entail a cost for production which ultimately leads to the disadvantage of a greater cost for the resulting photo-detectors. Thus, although the photodetectors using the conventional inorganic cut filters show spectral sensitivity approximate to the specific visibility curve, they are disadvantageous from the standpoint of reduced working performance, increased cost for production, and difficulty in improving the production process.
In cases where complexes are used as an infrared absorbent for near infrared absorbing plastic films, difficulty in obtaining thin film formation is encountered due to their low solubility in organic solvents.
In the above-described applications of an infrared absorbing composition such as, for example, filters for an SPD, there is a demand for compositions to be formed in a very thin film and have high infrared absorption efficiency. To this effect, the composition should comprise a resin having dispersed therein a large quantity of an infrared absorbent. Therefore, the conventional infrared absorbents having low solubility in organic solvents do not meet this purpose.
In addition, since the near infrared absorbing plastic films containing metal complexes show a maximum absorption in a shorter wavelength, they are unsuitable for the increasing use of infrared absorbing plastic films in photoreceptors of semi-conductor lasers.